1. Field Of The Invention
The field of the present invention generally relates to a hold-down device for securing an electrical connector. More particularly, the field of the present invention relates to a resilient hold-down device for repeated mounting of a connector to a printed circuit board, microprocessor board, or the like.
2. The Prior Art
The conventional method of mounting an electrical connector to a printed circuit board is by means of a post provided integrally with the lower surface of the insulating housing of the electrical connector. For example, when the insulating housing is comprised of a plastic material, the prior art hold-down device consists of a post, integrally formed from the plastic housing. The hold-down function is achieved by an interference fit between the post and a corresponding insert opening on the printed circuit board. The connector must be pressed downward so that the plastic post fits into a corresponding opening in the printed circuit board in order to secure the connector on the printed circuit board. Although the prior art interference fit hold-down device can mount an electrical connector tightly on the printed circuit board, such an interference fit plastic post may present problems.
Among the drawbacks of an interference fit device is the limited dimensional tolerance allowed between the post and the corresponding aperture on the printed circuit board. The problem of maintaining the precisely allowable tolerance between the post and opening is such that even modest variation in the dimension of post or insert opening may cause insufficient retention capability and may result in an unreliable electrical connection between the connector and printed circuit board. Even a slight dimensional variation also may result in excessive interference between the post and opening. This makes it difficult if not impossible to insert the post into the printed circuit board without danger of breaking off or fracturing the post and thereby rendering the entire connector useless.
In order to avoid the foregoing problem, it is necessary to impose significant manufacturing constraints to prevent dimensional variations from occuring during the manufacturing process. This in turn has the disadvantage of significantly increasing the difficulty and cost of manufacturing.
Another disadvantage of a typical interference fit hold-down device arises from the dissimilar thermal expansion coefficients between the integrally molded plastic mounting post and the printed circuit board. When the thermal expansion coefficients of the plastic mounting post and the printed circuit board are sufficiently different, the insert opening on the printed circuit board may apply a force to the free end of the post and cause fracturing or failure of the mounting post.
Another problem of a conventional hold-down device molded from plastic material is that plastic is easily scratched or otherwise subject to abrasion. Also, the conventional hold-down post tends to crack easily during an inappropriate insert-extract operation due to the lack of flexibility of the plastic or the insulating material. A plastic mounting post is also subject to failure after it has been used repeatedly to mount an electrical connector on a printed circuit board. In particular, when a connector is subjected to frequent use involving repeated cycles of insertion and extraction, the risk of failure of the plastic post is greatly increased.
What is needed therefore is a mounting means or hold-down device for securing an electrical connector to a printed circuit board which is not subject to breaking, fracturing or other structural failure. It would be desirable to form a hold-down device from a material having resilient characteristics which could be used over many cycles of insertion and extraction without failure. Because most electrical connectors utilize insulating materials which are not made from resilient materials, it is desirable to manufacture separately a mounting or hold-down device having the desired resilient characteristics, and then insert the hold-down device into the insulator housing of the electrical connector after the electrical connector has been completely assembled and is ready for mounting.
U.S. Pat. No. 4,681,389 is an example of a conventional hold-down device which avoids the use of an integrally formed plastic post. This patent discloses a metal lock pin for mounting an electrical connector to a printed circuit board or the like. However, this device has a significant limitation because it must be inserted from above to hold an upper shoulder surface provided on the insulator body of the electrical connector. A portion of the lock pin then extends downwardly through the electrical connector to engage an opening of the printed circuit board.
This arrangement has limited application because it only can be used to secure an electrical assembly which has a laterally extending shoulder portion. The shoulder portion of the electrical connector is necessary to provide the retaining surface which enables the lock pin to hold the connector assembly in place. Most electrical connectors are simple rectangular blocks without shoulders. Adding a shoulder in order to provide a surface for securing the connector increases manufacturing costs and uses up valuable space on a printed circuit board layout. In addition, the shoulder of the electrical connector assembly is structurally weak because it must be thin or narrow in order to allow the legs of the lock pin to have sufficient space to engage the opening in the printed circuit board. In practice, the plastic shoulder of such an electrical connector is usually very thin and subject to structural failure when exposed to vibration, rough treatment or harsh operating conditions.
Another disadvantage of a conventional hold-down device exemplified by U.S. Pat. No. 4,681,389 is that all elements of the lock pin are aligned in a common plane and define a single longitudinal axis. Because the lock pin device is typically very thin, the single plane surface lacks stability when subjected to torsional or lateral forces.
In addition, the manufacturing process necessary to produce a typical electrical connector assembly housing capable of holding perhaps hundreds of pins is complex. It is therefore desirable to maintain extreme simplicity in the manufacturing of the connector housing. For this reason, electrical connector housings are usually formed as rectangular blocks. It would be difficult, in terms of manufacturing steps and increased cost, to manufacture a housing with a shoulder structure sufficiently large to provide a stable retaining surface for such a prior art lock pin device.
Also, any insertion difficulty in loading a conventional hold-down device creates a potential for fracturing or otherwise weakening the thin plastic shoulder. And, the typically thin plastic shoulder configuration is not adapted to multiple insertion/extraction operations. The lack of structural integrity inherent in such prior art hold-down devices potentially creates unreliability in the electrical connections and may shorten the useful lifetime of the electrical connector assembly.
Therefore, what is needed is an improved hold-down device for securing an electrical connector to a printed circuit board or the like. The hold-down device preferably should have a flexible structure and not be subject to breaking or fracturing. The hold-down device also should have flexibility to allow a greater tolerance in the attachment dimensions of the hold-down means and the insertion aperture in the printed circuit board. What is also needed is a hold-down device which can be easily manufactured and assembled in a variety of electrical connectors in order to reduce manufacturing complexity and costs. It would also be desirable to provide an electrical connector hold-down device with increased capability for multiple reinsertion in a printed circuit board. This would advantageously enhance the useful lifetime of an electrical connector incorporating such a hold-down device.